Mineral separation plants used in the titanium mineral processing industry world-wide consist essentially of similar process technologies applied in a manner that is often tailored to an individual ore bodies separation requirements. Dependent upon a wide number of factors including particle size and shape, mineral grade, geology of the ore body, type of mineral species present and the physical characteristics of said mineral species, a unique recovery process is applied to optimise plant performance and satisfy operational and capital cost targets. Nevertheless, all titanium mineral processing plants in the world utilise similar process technologies applied in varying ways to accomplish their process needs.
Mining is carried out by firstly excavating the ore and subjecting it to gravity concentration which isolates the heaviest particles into what is termed a heavy mineral concentrate. The heavy mineral concentrates are sent to a dry separation plant, where individual minerals species (of which there may up to 20 or more present) are separated using their different magnetic, electrical or other physical properties, often at elevated temperatures. Separation equipment commonly includes but is not limited to, high-tension electrostatic roll (HTR) and electrostatic plate (ESP) separators, as well as gravity and magnetic processes. Using electrostatic separation techniques the conductors such as rutile and ilmenite are separated from the non-conductors such as zircon, quartz and monazite. These separators are extensively used for the separation of conductor and non-conductor mineral species typically found in the titanium minerals industry.
A wide variety of electrostatic induced charge and ionised field separators have been invented over the last 90 years however the devices of existing commercial designs described below have undergone little fundamental change in recent years.
Based on the charging mechanisms employed, three basic types of “electrostatic” separators include; (1) high tension roll ionised field separators (HTR), (2) electrostatic plate and screen static field separators (ESP and ESS herein called ESP) and (3) triboelectric separators. ESP and HTR separators are the most commonly used today, although in recent times some interest has been directed towards triboelectric separators. However their application remains limited to mineral species that can be contact charged and so they are suitable for separations of non-conductor species only.
Customarily, HTR separators utilise a grounded roll that transports the feed material through the high voltage ionising field (corona) which charges the particles by ion bombardment. Conducting particles lose their charge to the earthed roll and are thrown from the roll by centrifugal and gravity forces. Non-conducting particles are pinned to the rotor and are transported further around the roll before their charge either dissipates and they are thrown off or are removed by either mechanical means (brush) or high voltage AC wiper.
ESP separators have an electrode designed to generate a static field and the particles are charged by conductive induction. In their common form ESP separators utilise a stationary grounded surface such as a plate over which the material flows, forming the connection to ground that particles must have to allow them to become charged by induction. Triboelectric separators do not use an electric field to effect particle charging. Particle to particle and/or particle to surface charging occurs when particle species with different contact charging potential are brought into contact with one another. The particle charge attained can then be utilised to effect a separation in a static electric field.
These three basic separation types are often not present alone in any mechanism, and the machine characterisation essentially refers to the predominant or major separating effect. The present invention relies primarily on ion bombardment to charge the particles and so the operation of a HTR separator is described in more detail below.
The main separating mechanism employed in HTR separators involves the fact that conductors will quickly release their charge to a grounded surface and accordingly will be thrown off the rotating roll surface due to the centrifugal or gravitational forces. Non-conductors being unable to conduct their charge to the grounded surface are pinned to the roll surface. An “image force” pins the non-conductors to the roll and it can be shown that the image charge developed on the conducting surface is related to the particle charge and its distance from the roll surface. If the particle charge is negative, it repels electrons in the image vicinity in the conducting roll i.e. it generates its own positive image. This image has opposite polarity and the particle is attracted and pinned to the roll surface for this reason.
Thus the conductors tend to be thrown off the roll surface by their natural gravitational and centrifugal forces before falling through a splitter type collection means below and/or beyond the roll, dividing the feed into a conductor rich fraction and a conductor poor, or non-conductor, fraction.
Individual particle mass and shape partially determine the behaviour of individual particles in the separator and also the path followed by a particular particle once it has left the roll surface.
The above description of the separation process describes a one-stage HTR separator. HTR separators typically incorporate up to 3 identical stages with up to two starts or individual streams being treated in one machine. Very simple separations such as removing highly conductive ilmenite from good non-conductors can often be effected with just one stage. Nevertheless, in a multi-stage machine each new stage follows the last with material cascading from one stage to the next. Conductor or non-conductor retreat configurations are common.
Each stage is similar to the first with feed chute, earthed roll, electrode and splitter system duplicated and arranged one above the other in a vertical configuration. Adjustment of splitters, electrode position and roll speed is typically done at each stage independently of other stages.
In the treatment of mixtures of particles with a range of physical characteristics including conductivity, particle size and density, it is necessary to accurately set roll speed and relative positions of the electrode and splitters to achieve effective separation. It is usually necessary to adjust not only the air gap between the roll and electrode but also the alignment of the wire electrode and its backing member relative to the roll surface as well as the splitter positions, independently on each stage.
It is found in conventional HTR separators that not all particles contact the roll for sufficient duration to enable the conductors to be discharged and thrown Some of the particles which are fed onto the roll bounce up upon contact with the roll, as it rotates at relatively high speed. This results in lower separation efficiency. In addition, feed streams containing particles with low conductivity, such as leucoxene, may be incompletely separated as a result of incomplete discharge. Furthermore, feed streams in which there is wide particle size variation, particularly where the non-conductors are larger than the conductors, and feed streams containing fine particles below 75 microns in size may be incompletely separated. The present invention provides a means for minimising particle bounce and enhancing charge decay in conducting particles in order to improve separation efficiency.